A number of processes utilized in the pulp and paper industry perform poorly or even unsatisfactorily because air or gases are mixed or entrained in the liquid or liquid suspension--i.e. the filtrate paper stock or paper pulp. Pulp often contains more than 10% air which is bound in the fiber network, primarily in the form of small bubbles.
In the manufacture of pulp--specifically, in the sulfate process after the digestion step--there is produced a waste liquor also known as black liquor. Black liquor binds considerable amounts of air and the volume of air entrained in the black liquor is dependent on both the temperature and the concentration thereof. Consequently, problems arise as, for example, in the washing of the pulp after the digester.
It is known to remove air from pulp mixtures, e.g. to secure the operation of the following processes: air is removed during the washing of pulp by the addition of dispersing and antifoaming agents; air is removed from filler black liquor supplied to a digester by surface active agents; and air is removed in the washing plant by specifically designing the filtrate tanks so that the surface area of the fiber suspension is greatly enlarged.
In the pumping of pulp, it is known to utilize pumps wherein air is removed for the sole purpose of insuring the proper operation of the pump. Most of the pumps used for separating air utilize an external source of suction for removing air which accumulates in the pump. It is also known that air bubbles created in front or immediately upstream of the impeller can cause clogging of the normally free passage from the pump inlet to the pump outlet. In these cases the pump impeller has heretofore been provided with holes or openings allowing the air to pass through the impeller under the force of an additional external vacuum system attached to or located proximate the pump. Generally, the external vacuum system is composed of a separate liquid ring pump or the like. Pumps utilizing such vacuum systems for the sole purpose of insuring control and proper operation of the pump are known from U.S. Pat. Nos. 4,410,337; 4,335,193 and 4,273,562. Apparatus for degasification of liquids are also known from U.S. Pat. Nos. 3,686,831; 4,201,555 and 4,600,431. U.S. Pat. No. 3,597,904 discloses an apparatus for separating gas from a liquid and having an external vacuum source for deaerating fiber pulp suspensions fed to a paper-making machine. Finally, Swedish Patent No. 363,363 discloses a centrifugal pump having a vacuum pipe in the center thereof connected to an external vacuum source for air removal.
Thus, these known pumps-remove air from the pump for the purpose of preventing loss of pump action due to the accumulation of air within the pump. It is also recognized that air accumulates in and about the center of the rotating impeller of a centrifugal pump due to pump-generated centrifugal forces which carry the heavier substances such as liquid and solid particles of the medium to be pumped towards the periphery of the flow while the lighter components of the medium--i.e. the air or gases--collect at the center of the pump. The accumulated air impedes the pumping process and gradually prevents the medium to be pumped from flowing to the impeller. Known deaeration methods include the removal of air in medium consistency pumping in which the paper pulp is fluidized, the air-gas mixture in the fluidized stock is separated in a gas separation zone and the air or gas is removed in the degassing zone through use of a vacuum pump. The removal of air from low consistency pulp can also be accomplished by Assignee's AIRSEP system. The common feature of these known deaeration systems and methods is that they are employed for the sole purpose of assuring continual and proper operation of the pump.